VFX 101: Rectangle Inserts

(Herman Wang) #1

This is the fifth of a semi-regular series of columns I’ll be presenting on how to do some basic special effects, for people who have little to no experience. We use these effects quite frequently in our Harry Potter-based web series The Spell Tutor.

Today in the forum, someone was asking how to get a computer screen into a shot. As you may already be aware, when you film a screen directly you tend to get odd-looking swirly lines. This has to do with the mathematical relationship between camera frame rate and screen refresh rate, but I won’t get into a technical discussion here. Instead, I’ll demonstrate a technique that circumvents the problem.

We’ve used what I call “rectangle inserts” in The Spell Tutor a few times to get the “moving portrait” effect that Potter fans are familiar with. This technique works equally well for computer screens.

I’ll use this raw clip as the basis for this demonstration. There are several points worth noting about this clip:

  1. The camera is tripod-mounted and stays still for the entire clip.
  2. The screen similarly does not move during the clip.
  3. The actor never gets between the screen and the camera. This is critical for this technique to work.
  4. There happens to be a greenscreen mask in the television, but this is not a greenscreen technique.

After dropping the raw clip in my timeline, I’m going to select an image file to do my rectangle insert. (Note that you can just as easily use a clip instead of an image file.) The higher the resolution of what you select, the better. I’m also creating a mask so that the square image will match the rectangular ratio of the screen. I’m doing it by eye to start and I can always go back to fine tune it later.

I drop the image file into the timeline above the main clip and use positioning to shrink the overall size of the image, and now you can see why I call this a “rectangle insert”.

Because the television is set at a slight angle, you can see that the screen is no longer really rectangular as far as we’re concerned. The way we’ll compensate for that is to set the image layer to use 3D placement instead of the default flat 2D. In Vegas, this is done by open the track’s Track Motion window and changing the source setting to “3D Source Alpha”. I’m not familiar with how this is done in other editors - if you know please feel free to add a comment to this post.

3D placement allows me to adjust the rectangle to account for the slight angles in this shot. Most of the work here is to rotate on the Y axis to match the amount the television is turned.

Because of the static nature of the shot (as mentioned in points 1 and 2 above), this is really all you need to do. Here’s the final clip:

If you want a bigger challenge, you can try this technique where the camera is in motion. Here are a few samples of this from The Spell Tutor:
Episode 1.1
Episode 1.4
Episode 2.4

Screen capture versus filming a screen?
VFX 101: Logo Case Study
VFX 101: Shooting for Motion Tracking